The Telescope:

The following is a brief description of the 150-foot solar tower telescope. The basic design of the image-forming portion of the telescope has not changed in over 90 years of operation.

The Top of the Tower:

Located within the dome at the top of the tower are two flat mirrors (arranged in a coelostat design) and an objective lens. The diameter of the first flat is 19 inches, while that of the second flat is 14½ inches. Both mirrors are made of a Cervit substrate and polished flat to 1/20 of a wavelength of green light.
The first flat mirror is tilted at an angle parallel to the earth's axis of rotation and rotates about an imaginary line on its surface. The rate of rotation of this flat is exactly one half the rate of the sun's apparent motion through the sky. Thus, once aligned, light reflected off its surface is constantly directed onto the second flat mirror. This second flat is located above and to the south of the first flat and is oriented face down. The second flat directs light down to the objective lens below. This lens is an apochromatic cemented triplet manufactured by Perkin Elmer in 1971, following an optical design computed by Dr. Ira S. Bowen. Its nominal focal length is 150 feet, thus giving the tower its name. (The tower structure itself is approximately 176 feet high.) Electronic controls in the observing room subtly manipulate the position of the second flat allowing the observer to translate the image of the solar disk at ground level.

The Observing Room:

The observing room is located at the base of the telescope, some 150 feet below the level of the objective lens. The projected solar image formed by the two flat mirrors and objective lens is 415-430 mm in diameter, depending upon the time of year. Due to the telescope's long focal length, it is safe to view the image of the sun with the naked eye when projected onto the observing table. Every clear day, a solar observer creates a drawing of sunspots seen on the solar image.
The magnetic intensity of each individual spot (measured to the nearest 100 Gauss) as well as the position of each spot group is noted on each drawing. The database of daily drawings made at the 150-foot tower goes back to 1917 and contains close to 25,000 separate drawings.

Article by Tom Shieber.

Line drawings on this page have been adapted from the originals found in "Magnetic Observations of Sunspots: 1917-1924," Papers of the Mt. Wilson Observatory, Vol. V, Part 1, by George E. Hale and Seth B. Nicholson.